Current approaches to wood preservation on an industrial scale are based predominantly on the use of pressure and/or vacuum techniques for introducing especially fungicidal substances or other biocidal substances (possibly in combination with substances serving other functions, e.g., UV protectants) into certain woods, notably various species of pine (Pinus) and larch (Larix), as well as Douglas fir (Pseudotsuga menziesii; also known as Douglas pine, Douglas spruce, Oregon fir or Oregon pine), in which the cellular structure of the sapwood (alburnum) renders it receptive, under appropriate conditions, to impregnation with substances dissolved in liquid media. Heartwood, which often has a high content of natural resin (which itself normally confers a considerable degree of fungal rot resistance to the heartwood), is normally not accessible for impregnation to any significant extent.
Current industrial processes for impregnating wood may broadly be divided into two major classes, viz. pressure impregnation and vacuum impregnation.
In pressure impregnation processes, articles of wood (e.g., pine wood) to be impregnated are immersed, in an appropriate pressure vessel (tank or the like) in a solution comprising one or more impregnating substances dissolved in aqueous medium or in an organic solvent (depending on the nature of the impregnating substance(s)). The vessel is then pressurized (pressure typically in the range of about 1.5-10 bar), normally at a temperature in the range of 20-60.degree. C., for a period of time (typically from 15 minutes to 2 hours) which is adequate to ensure satisfactory penetration of the impregnation solution into the wood.
In vacuum impregnation processes, articles of wood to be impregnated are normally first subjected, in an appropriate vessel, to a reduced pressure for a period of time, after which the impregnation solution is admitted directly to the vessel so as to equalize the pressure and result in submersion/immersion of the wood articles therein in the solution.
The reduced pressure employed will normally be a pressure slightly above that at which boiling of the impregnation solution will occur at the temperature in question. In the case, for example, of impregnation solutions based on toluene and/or xylenes as solvent, a pressure of about 6-8 kPa at ambient temperature is fairly typical.
As with pressure impregnation, immersion is continued for a period of time sufficient to ensure adequate penetration of the impregnation solution into the wood.
Typical aqueous impregnation media which have been employed in pressure or vacuum impregnation include aqueous solutions of water-soluble substances, particularly inorganic substances such as copper salts, chromium salts, arsenic compounds, phosphorus compounds, boron compounds and/or fluorides.
Typical non-aqueous impregnation media which have been employed in pressure or vacuum impregnation include solutions of organic substances {e.g., coal-tar fractions (such as "creosote oil"), or halogen-containing aromatic compounds, such as pentachlorophenol or "dichlofluanide", i.e., 1,1-dichloro-N-((dimethylamino)-sulfonyl)-1-fluoro-N-phenylmethane-sulfena mide)} and/or organometallic substances (e.g., tin compounds such as bis(tributyltin) oxide ("TBTO") and/or tributyltin naphthenate ("TBTN")) in organic solvents. The use of a number of organic substances which were previously widely used for impregnating wood, such as pentachlorophenol and certain coal-tar fractions, is now banned in numerous countries.
Both pressure and vacuum impregnation techniques have been widely used to treat ready-made wooden articles (e.g., posts, telephone poles, garden furniture, doors and door frames, windows and window frames, fencing, components for construction of harbors, piers, etc.). The choice of, and amount of, fungicidal impregnating substance(s)/medium used to impregnate a particular type of article depends largely on whether the article is to be permanently in contact with, or may be brought into prolonged contact with, soil (i.e. earth), water or sea water, or is to be exposed to less drastic conditions, such as ambient weather conditions (intermittent rain, snow, wind, etc.).
Current non-industrial approaches to wood preservation predominantly involve application to the wood--e.g., by painting, spraying or dipping--of water-based or organic solvent-based commercial preparations containing fungicides (e.g., certain of those mentioned above), waxes, pigments, UV filters and the like.
In the case, in particular, of many of the water-soluble fungicidal substances (e.g., copper salts, chromium salts and arsenic-containing salts) used to impregnate wood (particularly pine wood), it has widely been believed that the active substance(s) undergo strong fixation in the wood primarily via formation of essentially water-insoluble substances within the wood by reaction (e.g. metathesis) of the water-soluble, active components (usually ions) with substances (such as relatively high-molecular-weight anionic or cationic species) which are naturally present in the wood.
In the case of numerous impregnating substances (particularly fungicides) which are soluble in organic solvents, it appears to have been more or less assumed that fixation of the active substance(s) in wood is ensured as a result of the essential insolubility thereof in aqueous media (e.g. rain water).
There is growing environmental concern in relation, in particular, to the use of non-biodegradable, toxic and/or ecologically damaging substances, such as heavy metal species (e.g., copper, chromium or tin species) and arsenic-containing species, as biocides for wood impregnation. In this connection there is now weighty evidence to indicate that fixation of, for example, arsenic-containing fungicidal species in wood (notably pine wood) is poor, and that significant leaching of these and other impregnating agents from impregnated wood can occur under some of the conditions (e.g. frequent exposure to rainfall, prolonged submersion in water and/or prolonged contact with moist soil) to which wood treated in this manner is frequently exposed.
Likewise, there is growing pressure to limit the use of numerous types of organic solvents, particularly hydrocarbon-type solvents such as those (e.g., toluene or xylenes) often employed in impregnation processes.
There is thus an increasingly pressing need for alternative methods and systems which can provide satisfactory protection of wood-based products against various forms of degradation, but which--as far as possible--avoid the undesirable environmental consequences associated with current approaches to wood preservation.
The present invention makes a significant contribution to the fulfilment of this need. Additionally the invention provides a new process for adding and fixating color(s) to solid wood or laminated solid wood.